Styrene monomer (SM) as an alkenylaromatic compound is typically produced through dehydrogenation of ethylbenzene, this production increasing year after year for using said SM as a raw material monomer for synthetic rubbers, ABS resins, polystyrene, etc. The dehydrogenation reaction of ethylbenzene is an endothermic reaction accompanied by volume expansion as represented by reaction formula (1) below, and is generally performed in the form of a mixture of ethylbenzene gas and water vapor (steam) under elevated temperature.C6H5.C2H5→C6H5.C2H3+H2−113 kJ/mol  (1)
Typically, the operation is performed with the inlet temperature of a catalyst bed being maintained to 600° C. to 650° C., although the temperature depends on the reactor and reaction conditions.
Such a method for producing styrene monomer has become technically feasible by virtue of the finding that iron oxide-potassium oxide catalysts (Fe—K catalysts) are less likely to be poisoned by steam, and consistently maintain a high performance. However, the catalyst needs to have a wider variety of performances according to the growing needs mentioned above.
The following are examples of important performance requirements for a catalyst for dehydrogenating ethylbenzene:    1) as low an operation temperature as possible;    2) high production yield of styrene;    3) low degree of catalyst deactivation due to, for example, carbon deposit;    4) the shaped product of the catalyst having sufficient mechanical strength to withstand stress during reaction; and    5) low production cost.The production yield of styrene is calculated as the product of the conversion and the selectivity.
The Fe—K catalyst described above is characterised in that it meets the performance requirements 1) as low an operation temperature as possible and 2) high production yield of styrene, and it is widely known that a composite oxide (KFeO2), generated from the reaction of Fe—K, which functions as an active species provides such features. With regard to lowering the activity due to carbon deposition in relation to 3), it is believed that potassium oxide maintains the activity through the facilitation of water gas shift reactions between the carbonaceous substance deposited on the surface of the catalyst and steam. Attempts to add an additional element to the Fe—K system to further improve its performance with respect to 1) and 2) above have been made, and, for example, the addition of an element such as Ce, Mo, Ca, Mg, Cr or the like to the Fe—K system is considered to be preferable for improvement of the activity (Patent Literature 1). Furthermore, improvements to the method for producing a catalyst, for example, improving the method for doping the third and fourth components to be added to iron oxide, have been attempted to improve the selectivity of a catalyst as much as possible (Patent Literature 2).
When a catalyst is shaped into pellets or the like and then installed in a reaction apparatus, the pellets are deformed by high temperature/high pressure stress applied to the catalyst in reaction, and interrupt the flow of reaction gas or the like to result in the suspension of the operation in the worst-case scenario. It is known that, to obtain the mechanical strength required to withstand the stress, the use of cerium carbonate hydroxide or a mixture of cerium carbonate hydroxide and cerium carbonate as a cerium raw material is particularly preferred (Patent Literature 3).
For the reason as described above, the development of a catalyst material has been heretofore conducted with the focus on the Fe—K—Ce system, and further improvement has been made through the addition of an element such as Mo and Ca, as described above, and as a result the Fe—K—Ce system has become widely used in the production process for styrene. However, the market is strongly demanding a lower cost and environmental friendliness, and thus further improved catalysts are required. One goal of such an improvement is the achievement of further reduced energy consumption and higher yield based on 1) and 2) above, without impairing 3) and 4) above.
In relation to the fact that the dehydrogenation of ethylbenzene is an endothermic reaction, there is a high demand for feature 1). The reason for this is that the outlet temperature of a catalyst bed in an actual reactor for production of styrene is under the inlet temperature by approximately 50 to 100° C., although the difference depends on reaction conditions, and thus maintenance of high activity not only at the inlet temperature of the catalyst bed (e.g. 600 to 650° C.) but also at the lower outlet temperature of the catalyst bed (e.g. under 600° C.) leads to a high yield on the whole of the reactor.
In Patent Literature 2, an improved yield is achieved through the improvement of the selectivity by adding third and fourth elements. However, no reference is made to the activity in a low-temperature region expected in the outlet of a reactor, and in Examples the reaction temperature is rather elevated on purpose to achieve a conversion of 70%, and the activity at low temperatures is not clear. Thus, whether the yield on the whole of an actual reactor is high is unclear.
For another example of attempts to improve the performance of a catalyst by adding third and fourth elements thereto, Patent Literature 4 demonstrates that additions of less than 300 ppm of Ti to an Fe—K—Ce material prevents the catalyst from being deactivated, and this allows the achievement of a conversion through a smaller rise in the reaction temperature. Even in Patent Literature 4, however, no reference is made to the activity in a low-temperature region expected in the outlet of a reactor, and in Examples the reaction temperature is rather elevated on purpose to achieve a conversion of 70%, and the activity at low temperatures is not clear.
Even in the case that adding third and fourth elements, as shown in Patent Literature 2 and Patent Literature 4, is found to improve the activity, other properties, for example, the mechanical strength of the catalyst may deteriorate depending on the type and quantity of the element added, and adequate care is needed to improve productivity in a practical way.
Although catalysts for dehydrogenizing ethylbenzene which cause less carbon deposits in reaction and provide a high yield in a high-temperature region have been heretofore proposed as described above, no catalyst which exhibits high activity even in a low-temperature region corresponding to the outlet temperature of a catalyst bed in an actual reactor and provides a high yield on the whole of a reactor has been proposed yet.